Redox amplification processes have been described, for example in GB Publications 1,268,126; 1,399,481; 1,403,418; and 1,560,572. In such processes color materials are developed to produce a silver image (which may contain only small amounts of silver) and treated with a redox amplifying solution (or a combined developer/amplifier) to form a dye image.
The developer-amplifier solution contains a color developing agent and an oxidizing agent that will oxidize the color developing agent in the presence of the silver image which acts as a catalyst.
Oxidized color developer reacts with a color coupler to form the dye image. The amount of dye formed depends on the time of treatment or the availability of the color coupler and is less dependent on the amount of silver in the image than is the case in conventional color development processes.
Examples of suitable oxidizing agents include peroxy compounds including hydrogen peroxide and compounds that provide hydrogen peroxide, e.g., addition compounds of hydrogen peroxide such as perborates and addition compounds of hydrogen peroxide with urea. Other oxidizing agents include cobalt (III) complexes including cobalt hexammine complexes; and periodates. Mixtures of such compounds can also be used.
The image-forming step can be followed by a stop bath, bleach and fix step, although the bleach and/or fix may be omitted if the silver coating weight of the material processed is low enough. When a bleach-fix is employed after redox amplification, the solution needs only small amounts of oxidizing agent such as iron (III) and fixing agent such as thiosulfate because there is only a small amount of silver to remove.
Recently it has been proposed that the bleach bath may contain a peroxide as sole bleaching agent. Such proposals have carried the warning that, at low pH levels, redox amplification should be stopped before bleaching otherwise there is a risk that color staining may occur due to image formation continuing in the peroxide bleach solution.
GB-2303930A describes a process in which a development step is followed by a bleach amplifier step. In this process at least 50% of the dye image is formed from the carryover of color developing agent from the developer solution to the beach amplifier bath. The bleach amplifier is of limited stability at pH ranges of 9.0 to 12.0 and decomposition of hydrogen peroxide occurs, eventually leading to loss of amplification. For example, during overnight standing the seasoned bleach amplifier will sometimes decompose and will then be unsuitable for further processing because of the risk of forming a yellow stain (not a dye stain) on the photographic material being processed.